Many new drugs are now available to be used by oncologists in treating patients with cancer. Often, tumors are more responsive to treatment when anti-cancer drugs are administered in combination to the patient than when the same drugs are administered individually and sequentially. One advantage of this approach is that the anti-cancer agents often act synergistically because the tumors cells are attacked simultaneously with agents having multiple modes of action. Thus, it is often possible to achieve more rapid reductions in tumor size by administering these drugs in combination. Another advantage of combination chemotherapy is that tumors are more likely to be eradicated completely and are less likely to develop resistance to the anti-cancer drugs being used to treat the patient.
One serious limitation of combination chemotherapy is that anti-cancer agents generally have severe side effects, even when administered individually. For example, the well known anti-cancer agent taxol causes neutroperia, neuropathy, mucositis, anemia, thrombocytopenia, bradycardia, diarrhea and nausea. Unfortunately, the toxicity of anti-cancer agents is generally additive when the drugs are administered in combination. As result, certain types of anti-cancer drugs are generally not combined. The combined toxic side-effects of those anti-cancer drugs that are administered simultaneously can place severe limitations on the quantities that can be used in combination. Often, it is not possible to use enough of the combination therapy to achieve the desired synergistic effects. Therefore, there is an urgent need for agents which can enhance the desirable tumor attacking properties of anti-cancer agents without further increasing their undesirable side-effects.
It has now been found that certain bis[thio-hydrazide amide] compounds significantly enhance the anti-cancer activity of taxol. For example, Compound (1) was used in combination with taxol (Paclitaxel) to treat tumors induced in nude mice from the human breast tumor cell line MDA-435. The tumor volume was about five fold less after 24 days of treatment in mice which had been administered 5 mg/kg of taxol and 25 mg/kg of Compound (1) than in mice which had only been administered 5 mg/kg of taxol or in mice which had only been administered 50 mg/kg of Compound (1) (Example 13). These results are shown graphically in FIG. 1. The structure of Compound (1) is shown below: 
It has also been found that these bis[thio-hydrazide amide] compounds have minimal toxic side effects. For example, the mice treated with taxol and Compound (1) showed little if any weight loss over the treatment period (see FIG. 2). Based on these results, novel compounds which enhance the anti-cancer activity of taxol, pharmaceutical compositions comprising these compounds and methods of treating a subject with cancer are disclosed herein.
One embodiment of the present invention is a compound represented by the Structural Formula (I): 
Y is a covalent bond, a phenylene group or a substituted or unsubstituted straight chained hydrocarbyl group. In addition, Y, taken together with both  greater than Cxe2x95x90Z groups to which it is bonded, is a substituted or unsubstituted aromatic group. Preferably, Y is a covalent bond or xe2x80x94C(R7R8)xe2x80x94.
R1 and R2 are independently an aryl group or a substituted aryl group, R3 and R4 are independently xe2x80x94H, an aliphatic group, a substituted aliphatic group, an aryl group or a substituted aryl group.
R5-R6 are independently xe2x80x94H, an aliphatic group, a substituted aliphatic group, an aryl group or a substituted aryl group.
R7 and R8 are each independently xe2x80x94H, an aliphatic or substituted aliphatic group, or R7 is xe2x80x94H and R8 is a substituted or unsubstituted aryl group, or, R7 and R8, taken together, are a C2-C6 substituted or unsubstituted alkylene group.
Z is xe2x95x90O or xe2x95x90S.
In one aspect, R1 and R2 in the compound represented by Structural Formula (I) are not both phenyl when Y is xe2x80x94C(R7R8)xe2x80x94, R3 and R4 are both phenyl and R5-R8 are all xe2x80x94H.
Another embodiment of the present invention is a pharmaceutical composition comprising a pharmaceutically acceptable carrier or diluent and a compound represented by Structural Formula (I). Preferably, the pharmaceutical composition comprises an effective concentration of the compound.
Yet another embodiment of the present invention is a method of treating a subject with cancer. The method comprises administering to the subject an effective amount of taxol or a taxol analog and an effective amount of a compound represented by Structural Formula (I).
The disclosed compounds increase the anti-cancer activity of taxol and taxol analogs. In addition, these compounds have minimal toxic side-effects. Consequently, it is possible to increase the effectiveness of taxol and analogs thereof when used in combination with the disclosed compounds, even when approaching the highest tolerated doses of taxol. Thus, it is expected that combination therapy with the compounds of the present invention will provide improved clinical outcomes for patients with cancers that are being treated with taxol. By co-administering the disclosed compounds with taxol, it is also possible to achieve the same therapeutic effectiveness previously achieved with higher doses of taxol, thereby reducing the side-effects and improving the quality of life for the patient.